Tohatsu 25hp sparking problem

I have an old 25hp Tohatsu two stroke engine, think it is the M25C series - no serial number - from early 80's I assume. It is a two cylinder single carb model with combined CDI/coil pack and hand start.

Problem is the top piston refuses to fire hence the engine not revving up as it should . Checked the reed valves and they are OK. Compression is good and same in both cylinders, new spark plugs fitted.
What happens is this: With the motor running and one pulls off the top plug wire, no difference. Pulls off the bottom plug wire and it dies immediately.
Swopping the plug wires around and starting the engine, it runs the same. Pulling off the wire on top cylinder, again no difference, pulls off the bottom one, the engine stops immediately again. This tell me the CDI is firing on both plug wires.

What mystifies me is the fact that it refuses to spark the top cylinder, even with plug leads changed around from CDI. Strangely, although the top plug lead fires the bottom cylinder when swopped around it will not fires the top cylinder - even when I test the lead with loose spark plug, it refuses to spark that, but will run the bottom cylinder on engine...

I checked the secondary windings resistance on the plug leads at spark plug ends and get same readings of 2782 ohms on each one. Checked all wiring from pickup coil to CDI and no bad places or joints.

Lastly, when I changed the plug leads back after I switched them around the motor fired on both cylinders for two seconds or so and went back in bottom cylinder firing alone.

What am I missing? Can anyone please point me towards a solution because Im desperate to get the problem solved.

 

Are you sure the top cylinder is getting fuel ? Are both coils good ?

 

Narrow your focus by eliminating the fuel or ignition side. Check for a good, clean, bright spark on the top plug. If you have a nice spark, it's obviously a fuel delivery issue, if not, it's ignition. If you just want to fool around with it, swap coils and see if the top comes to life.

 

In my OP I did note that I switched the coil wires (plug leads) around and only the bottom cylinder fires still up, and the top stays dead meaning the top cylinder coil is OK by firing bottom cylinder - the coil type is a combined two lead coil integrated into the CDI.

As for fuel - I removed the reed valve box and inspected reeds - all seal properly and opens freely. It is only a single carburetor engine and if the bottom cylinder get fuel, so must the top.

The strange is the fact that the top cylinder coil lead will not even spark a loose spark plug when hold (earthed) against cylinder top, but will fire the bottom cylinder

Can it be sucking air into to top cylinder casing at the top crank seal and not getting fuel to piston? But that does not explain the lack of spark holding loose spark plug against top...

 

Check compression. Switching the plug leads about surely would cause mis-fire ? Carby has only one throat, or two ? You are getting closer to the answer.....

 

single throat carb.
Compression actually slightly better on the problematic top cylinder.

With 2 cylinder on two stroke I believe one can turn around the leads will no ill effects.
This engine has only on pick-up coil (with exciter coil) under flywheel and has four magnets with gaps at every 90 degrees - this tells me it is a hall square wave system and fires every 90 degrees, in other word uses a wasted spark system if I read it correctly.

 

Too clever for me, I'd have thought an extra spark would ignite the fresh charge. It should not be too difficult to establish whether the top cylinder is sparking, just get someone to pull the starter while you ground the plug on the block. As PAR says, eliminate one first, that would be the ignition, imo, you have in two-strokes both primary and secondary compression, the latter can read OK but the former be well down or gone, because the sealing of the crankcase is faulty. The ignition is much easier to eliminate first.